Substance-sensitive electrical structures

ABSTRACT

Disclosed is a substance-sensitive semiconductor and a method for making the same, wherein a substance-sensitive material is combined with photoresist material and applied to an electronic device structure. The substance-sensitive material may be applied before or after the photoresist material, or even may be combined with the photoresist material to form a substance-sensitive layer of photoresist material on the semi-conductor. The photoresist material is then processed, such that unwanted, or undesirable areas are free from the photoresist material and the areas of desired substance sensitivity have a fully processed photoresist layer. A further embodiment of the present disclosure provides multiple layers sensitive to different ions on a single sheet of semiconductor or electromagnetically active material.

This is a division, of application Ser. No. 858,906, filed Dec. 8, 1977,now U.S. Pat. No. 4,302,530,

BACKGROUND OF THE INVENTION

The present invention relates generally to electronic andelectromagnetic devices for the determination of the presence andstrength of selected substances which include ions, molecules andligands.

Often times it is advantageous to detect the existence and/orconcentration of substances in the atmosphere or solutions. Generallysuch detectors involved exposure of a chemical compound to the testmedium with the indication of existence and/or concentration beingprovided by a color change, the formation of a precipitate, etc.

In the recent past, however, it has become possible to providesemiconductors which are sensitive to the exsistence of specificsubstances in liquids or gases, and provide an electrical indication ofboth the presence and concentration therein. Examples of these are theion-sensitive field effect transistor (ISFET), one example of which isshown in U.S. Pat. No. 3,831,432, and the ion controlled diode disclosedin co-pending U.S. patent application Ser. No. 781,474, filed Mar. 25,1977, herein incorporated by reference. Both devices incorporate an ionsensitive membrane which forms a charge layer near the semiconductorjunction which is dependent upon the concentration of the ion, moleculeor complex, whose concentration is to be determined. Thesubstance-sensitive material varies widely in its individual makeupdependent upon the particular substance to which the material issensitive. Many different materials can be used in the membraneconstruction in order to achieve varied substance-sensitivity. Forexample, one such substance-sensitive material which is sensitive topotassium ions (K⁺ ions) is valinomycin. Some other substance-sensitivematerials are listed in Membrane Electrodes by N. Lakshminarayanaiah,Academic Press, 1976, also herein incorporated by reference.

In the prior art substance-sensitive devices, the substance-sensitivematerial is formed into a cast membrane which is then located over thesemiconductor device. Alternately, the semiconductor device can becoated with the appropriate material. While these methods of forming thesubstance-sensitive membrane are suitable for a single device capable ofproviding an electrical indication of the concentration of a singlecomplex, it is extremely difficult to utilize cast membranes, orsubstance-sensitive coatings, to render only discrete portions of aLarge-Scale Integrated (LSI) circuit involving semiconductors, microwavestriplines or integrated optical structures sensitive to the substance.Thus, in the common mass production of integrated circuits, it isextremely difficult, if not impossible, to provide appropriate amplifiercircuitry along with a substance-sensor in a mass-produced integratedsemiconductor, microwave or optical circuit.

The difficulty of obtaining precise placement (measured in microns) of acast membrane, eliminates the applicability of cast membranes to LSIstructures. If existing LSI structures are coated withsubstance-sensitive materials, it would be possible to detect only onesubstance, and not a plurality of different substances.

SUMMARY OF THE INVENTION

Therefore, it is an object of the present invention to provide asemiconductor, microwave, or optical device which is sensitive andprovides an electrical indication of the existence and/or concentrationof a predetermined substance, which device can be produced in accordancewith existing mass production techniques.

It is a further object of the present invention to provide a method offorming a substance-sensitive semiconductor, microwave or optical deviceunder current mass production technology.

It is a still further object of the present invention to provide amethod of forming substance-sensitive layers on semiconductors,microwave or optical devices which are sensitive to different substancesutilizing current mass production technology.

An additional object of the present invention is to provide asubstance-sensitive semiconductor having a substance-sensitivephotoresist layer which is compatible with large-scale integratedcircuit technology.

It is a still further object of the present invention to provide asubstance-sensitive semiconductor which incorporates multiplesubstances-sensitive sensors.

The above, and other objects, are achieved by the method ofincorporating substance-sensitive material with photoresist material,and applying the combination to the required portion of a semiconductor.The photoresist material is masked, exposed to radiation, etched andchemically treated to leave a substance-sensitive photoresist layer onlyon selected portions of the semiconductor material. In preferredembodiments, a plurality of photoresist layers sensitive to differingsubstances can be provided on adjacent portions of a large scaleintegrated circuit chip.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention, and the attendantadvantages thereof, will be more clearly understood by reference to thefollowing drawings, wherein:

FIGS. 1a-1j are partial, cut-away views of semiconductor materialshowing the steps utilized in preparing two substance-sensitivemembranes, each membrane sensitive to a different substance;

FIG. 2 is a schematic view of a capacitive application of thesubstance-sensitive layer; and

FIG. 3 is a schematic view of an inductive application of thesubstance-sensitive layer.

FIG. 4 shows a typical ISFET in which the present invention may beapplied.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The term photoresist material applies to a large number of commerciallyavailable materials which are used in the preparation of semiconductorcircuitry. The unique property of photoresist material is that exposureto radiation causes, or prevents, polymerization of the photoresistmaterial, such that the unpolymerized material can be later removed fromthe device to which the photoresist material, as applied to the presentinvention, incorporates all materials which are sensitive to radiationexposure (whether it be visible light, ultraviolet, infrared, X-ray,electron beam, ion beam, etc. radiation) when the activation (exposure,or non-exposure) prevents, or causes, a change in the etching rate orpolymerization in the material itself. It is this property which enablesthe present method to achieve the very precise edge definition tosubstance-sensitive membranes which has theretofore been impossible withcast, or coating, type membranes.

As noted earlier in the Background of the Present Invention, there are anumber of materials which are sensitive to the presence and/orconcentration of various substances (ions, molecules, ligands and otherchemical groups). Because the etching or removal rate of photoresistmaterial changes when activated (by exposure, or non-exposure, toparticular radiations depending upon the type of photoresist), smallquantities of substance-sensitive materials can be dissolved, and then,fixed in the photoresist material by the activation process. If aphotoresist material is doped with a substance-sensitive material, andsubsequently activated, the result is a substance-sensitive layer whichwill remain on the surface of the structure to which the photoresistmaterial is initially applied.

Becuase the combination of photoresist material and substance-sensitivematerial produces a material whose remaining existence on a surface isdependent upon whether or not it has been exposed to the radiation ornot, extremely sharp and precise definition can be given to the edges ofphotoresist material having the substance-sensitive material suspendedtherein. Thus, substance-sensitive photoresist layers can be placed onsurfaces with an accuracy measured in microns, permitting theapplication of a plurality of extremely accurate chemical sensors to beproduced on a single substrate.

The negative photoresist is activated when exposed to the appropriateradiation, and the positive-type photoresist is activated unless it isexposed to the appropriate radiation.

It is understood that the term "processing" encompasses the steps ofselectively activating and removing substance-sensitive photoresistmaterial to leave a substance-sensitive photoresist layer at adesignated location, said steps dependent upon the type of photoresistmaterial chosen.

By the same token, layers of substance-sensitive photoresist can beapplied to the same substrate, each layer being sensitive to a differenttype of substance. An example of the inventive method applied to a twosubstance-sensitive semiconductor is shown in FIGS. 1a-1j.

A semiconductor substrate 10 is shown in FIG. 1a, comprising P-type andN-type semiconductor materials. Because several substance-sensitiveelements are going to be placed on the single substrate, a plurality ofP/N junctions are provided. A passivation layer 12 is applied to thesubstrate, with the exception of the prepared places 14 and 16 where themembrane layers are to be placed. The prepared places may include ablocking layer to prevent either electronic or ionic conduction, or massdiffusion, as necessary. The passivation layer may comprise silicondioxide (SiO₂), or any other suitable isolation material known to thosein the art.

A first layer of substance-sensitive photoresist material is appliedover prepared places 14 and 16, as well as the passivation layer 12. Thesubstance-sensitive photoresist layer can be applied by first coatingthe passivation layer, and prepared spaces, with a layer ofsubstance-sensitive material, over which is placed a layer ofphotoresist material, such that the substance-sensitive diffuses intothe photoresist layer. Alternatively, a layer of photoresist materialcan be applied with a coating of substance-sensitive material thereover.The substance-sensitive material is permitted to diffuse into thephotoresist material, forming a substance-sensitive photoresist layer.In a preferred embodiment, a substance-sensitive material is first mixedwith the photoresist material, and then, applied to the substrate as asubstrance-sensitive photoresist layer.

The substrate, covered with the substance-sensitive photoresist layer18, is covered by a suitable mask 20, and subjected to an appropriateradiation indicated by arrows 22. As is known, photoresist materialcomprises two primary groups, positive and negative photoresist. In thisinstance, a negative-type polymerizable photoresist material isdepicted, with an appropriate mask 20 to polymerize thesubstance-sensitive photoresist material in the vicinity of preparedspace 14. If a positive-type photoresist material were used, the maskwould cover only the vicinity of prepared space 14, allowing the rest ofthe photoresist material to be exposed to radiation 22, thus, preventingits polymerization.

The unpolymerized substance-sensitive photoresist material is removed byconventional etching procedures, leaving a single substance-sensitivephotoresist layer 24 in the region of prepared space 14.

A second layer of substance-sensitive photoresist material 26 is placedover the substrate and isolation layer. However, the second layer ofsubstance-sensitive photoresist may be sensitive to a substancedifferent from that of said first substance-sensitive photoresist layer.As shown in FIG. 1g, a suitable mask 28 is placed over the coatedsubstrate, with arrows 22 indicating the radiation for polymerizing thephotoresist layer in the vicinity of prepared space 16. After suitableetching and processing steps, the substrate, as indicated in FIG. 1h,has a substance-sensitive photoresist layers 24 and 26, which areresponsive to different substances. After the addition of referenceelectrodes 30 and 32, and suitable contact electrodes 34, the chemicalsensor is ready for operational use, as indicated in FIG. 1j.

It should be noted that, although a multiple substance-sensitivechemical sensor has been shown in FIGS. 1a-1j, the procedure would beprecisely the same for making only a single substance-sensitivephotoresist layer, or for making a plurality of substance-sensitivesensitive photoresist layers which are sensitive to differentsubstances.

It is clear that many different types of photoresist material, bothpositive and negative, can be utilized in accordance with the presentinvention. The process of photopolymerization and the physical andchemical properites of a number of different photoresist materials arefurther discussed in Solid State Technology, June and September, 1971,and Electronic Components, June 29, July 27 and August of 1973, saidarticles herein incorporated by reference. For providing asustance-sensitive photoresist layer which is sensitive to potassium(K⁺) ions, valinomycin can be utilized, which is available from Cal-BioChem, East Rutherford, New Jersey. One specific photoresist materialwhich was used is type AZ 1350J, available from Shipley, Allentown, Pa.It is clear that many other types of photoresist material, both positiveand negative, can be utilized in accordance with the present invention.

Similarly, in order to obtain photoresist layers which are sensitive todifferent substances, different substance-sensitive material can beutilized as herein before noted. In a preferred embodiment in which thesubstance-sensitive material is mixed with the photoresist material andthen applied to the substrate, 10 mg. of valinomycin was dissolved in 10cc. of negative photoresist material. The substance-sensitivephotoresist material was spun onto a silicon wafer coated with a 6,000 Åthick layer of silicon dioxide, utilizing standard methods. Thesubstance-sensitive photoresist layer was prebaked at a temperature of50° C. for a short period of time and then exposed to ultravioletradiation in a standard mask aligner in order to polymerize thematerials. Two micron thick and thinner layers of substance-sensitivephotoresist material have been provided, which give the necessarysensitivity to the existence, and concentration, of potassium ions.

Instead of providing multiple layers on an ion controlled diode, asrepresented in FIGS. 1a-1j, the applicants ' inventive method could beapplied to providing substance-sensitive photoresist layers on ionsensitive field-effect transistors (ISFET) as taught in U.S. Pat. No.3,831,432. A typical ISFET structure is illustrated in FIG. 4. Althoughthe operation of the semiconductor device is different from that of anion controlled diode, the effect of the substance-sensitive photoresistlayer is similar, and the inventive method can be utilizedinterchangeably for ion controlled diodes, ISFET's or combinationsthereof.

In fact, the inventive method of the present invention can be applied toelectronic structures other than semiconductors. FIG. 2 illustrates theutilization of a substance-sensitive photoresist layer to vary thecapacitance between the reference electrode 40 and a base electrode 42.The test medium 44 (the material in which it is desired to sense thepresence, or concentration, of the desired substance) causes changes inthe dielectric constant of the substance-sensitive photoresist layer 24,when the desired substance is present. It is clear that a change indielectric constant will change the coupling and thus the capacitancebetween the plates of the capacitor. An isolation layer 41, upon whichthe photoresist layer is applied, serves to prevent the test medium fromshorting the capacitor plates or the photoresist layer. The above is apreferred embodiment, although one could easily locate the photoresistlayer on its own isolation layer at any point between the capacitorplates. These changes in the dielectric constant are reflected inchanges in capacitance betgween the reference electrode 40 and the baseelectrode 42. These changes in capacitance can be measured, such that adigital output indicative of the concentration of the unknown complex isprovided, as is shown in co-pending application Ser. No. 781,474, filedMar. 25, 1977.

Similarly, changes in permittivity and permeability in thesubstance-sensitive photoresist layer in response to the presence of thedesired substance, can be reflected in changes of the coupling betweentwo lines, as is shown in FIG. 3. An input line 50, and an output line52, are coupled in part by a substance-sensitive photoresist layer 24.Test medium 44 causes changes in the permittivity and permeability ofthe substance-sensitive photoresist layer, which ultimately causes achange in the coupling between the input line 50 and the output line 52,which change can be measured providing an indication of theconcentration of the desired substance. While the FIG. 2 embodimentwould have a D.C. or low frequency application, the FIG. 3 embodimentwould primarily apply to high frequency devices.

The electronic structures schematically depicted in FIGS. 2 and 3 may beapplied to microwave striplines and integrated optical structures, aswell as other electronic devices, which will become obvious to those ofordinary skill in the art in view of the above teachings. FIG. 4illustrates the application of a substance-sensitive layer 24 placed ontop of a blocking layer 60 separating a field-effect transistor fromtest medium 44. Therefore, it is believed that the present inventivemethod and apparatus is not limited to the specific embodiments hereindiscussed, and many modifications and variations thereof will be readilyapparent to those skilled in the art, in the light of the aboveteachings. It is, therefore, to be understood that, within the scope ofthe appended claims, the invention may be practiced otherwise than asspecifically described.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A substance-sensitivestructure for sensing the presence and/or concentration of a substancein a test medium, said structure comprising:structure means, responsiveto change in a dielectric constant, for indicating the concentration ofsaid substance in said test medium as determined by said change in saiddielectric constant; and a substance-sensitive layer for separating saidstructure means from said test medium and for changing dielectricconstant in response to the concentration of said substance in said testmedium, said layer comprised of substance-sensitive photoresist materialincluding a photoactivator.
 2. The structure of claim 5, wherein saidstructure means comprises a semiconductor diode having a P/N junctionwith said substance-sensitive photoresist material separating said P/Njunction from said test medium.
 3. The structure of claim 5, whereinsaid structure means is a capacitor having at least two plates, and saidsubstance-sensitive photoresist material is disposed between saidplates, and said test medium is exposed to said material.
 4. Asubstance-sensitive structure for sensing the presence and/orconcentration of a substance in a test medium, said structurecomprising:structure means, responsive to changes in a dielectricconstant, for indicating the concentration of said substance in saidtest medium indicated by said change in said dielectric constant; and asubtance-sensitive layer disposed in said test medium for changingdielectric constant in response to the concentration of said substance,said structure being a pair of lines coupled through saidsubstance-sensitive layer, said layer comprised of substance-sensitivephotoresist material.
 5. A substance-sensitive structure for sensing thepresence and/or concentration of at least a first substance and a secondsubstance in a test medium, said structure comprising:a first electronicstructure responsive to change in a dielectric constant for indicatingthe concentration of said first substance in said test medium by saidchange in said dielectric constant; a first substance-sensitive layerfor separating said first electronic structure from said test medium andfor changing dielectric constant in response to the concentration ofsaid first substance; a second electronic structure, integral with saidfirst electronic structure, responsive to change in a dielectricconstant for indicating the concentration of said second substance insaid test medium by said change in said dielectric constant; and secondsubstance-sensitive for separating said second electronic structure fromsaid test medium and for changing dielectric constant in response to theconcentration of said second substance, both of said substance-sensitivelayers being comprised of substance-sensitive photoresist materialincluding a photoactivator.
 6. The structure of claim 5, wherein saidstructure means comprises a field effect transistor and saidsubstance-sensitive layer is disposed on said field effect transistor.7. The structure of claim 6, wherein said field effect transistorcomprises:a semiconductor substrate; a pair of spaced-apart diffusionregions on said semiconductor substrate; a blocking layer covering saidsubstrate and spaced-apart diffusion regions; a layer ofsubstance-sensitive photo-resist material on said blocking layer; and areference electrode coupled to one of said diffusion regions, saidsubtance-sensitive layer separating said blocking layer from said testmedium and said reference electrode disposed in said test medium.
 8. Asubstance-sensitive structure for sensing the presence and/orconcentration of a substance in a test medium, said structurecomprising:means for changing dielectric constant in response to changesin concentration of said substance in said test medium; and means,responsive to changes in said dielectric constant, for indicating theconcentration of said substance in said test medium, said indicatingmeans including means for detecting the effect of said changes indielectric constant on a time varying electrical signal.
 9. Asubstance-sensitive structure for sensing the presence and/orconcentration of a substance in a test medium, said structurecomprising:structure means, responsive to specific changes, forindicating the concentration of said substance in said test medium asdetermined by said specific changes; and a substance-sensitive layer forseparating said structure means from said test medium and forspecifically changing in response to the concentration of said substancein said test medium, said layer comprised of a photoresist materialincluding a photoactivator and a substance-sensitive material.
 10. Thestructure of claim 9, wherein said structure means comprises asemiconductor diode having a P/N junction with said layer separatingsaid P/N junction from said test medium.
 11. The structure of claim 9,wherein said structure means is a capacitor having at least two platesand said layer is disposed between said plates, and said test medium isexposed to said material.
 12. A substance-sensitive structure forsensing the presence and/or concentration of a substance in a testmedium, said structure comprising:structure means, responsive tospecific changes, for indicating the concentration of said substance insaid test medium indicated by said specific changes; and asubstnace-sensitive layer disposed in said test medium for specificallychanging in response to the concentration of said substance, saidstructure means being comprised of a pair of lines coupled through saidsubstance-sensitive layer, said layer comprised of a photoresistmaterial including a sensitizer and a substance-sensitive material. 13.The structure of claim 9, wherein said structure means comprises afield-effect transistor and said substance-sensitive layer is disposedon said field-effect transistor.
 14. The structure of claim 13, whereinsaid field-effect transistor comprises:a semiconductor substrate; a pairof spaced-apart diffusion regions on said semiconductor subtrate; ablocking layer covering said substrate and spaced-apart diffusionregions; and a reference electrode coupled to one of said diffusionregions, said substance-sensitive layer separating said blocking layerfrom said test medium and said reference electrode disposed in said testmedium.
 15. A substance-sensitive structure for sensing the presenceand/or concentration of a substance in a test medium, said structurecomprising:structure means, responsive to a substance-sensitivephotoresist layer including a photoactivator, for indicating theconcentration of said substance in said test medium as determined bychanges in said substance-sensitive photoresist layer; and asubstance-sensitive photoresist layer for separating said structuremeans from said test medium and for changing in response to theconcentration of said substance in said test medium, said layer furtherincluding a substance-sensitive material and said layer furthercomprising the sole means for bonding said substance-sensitive materialto said structure means.
 16. The structure of claim 19, wherein saidstructure means comprises a semiconductor diode having a P/N junctionwith said substance-sensitive photoresist material separating said P/Njunction from said test medium.
 17. The structure of claim 19, whereinsaid structure means is a capacitor having at least two plates and saidsubstance-sensitive photoresist material is disposed between saidplates, and said test medium is exposed to said substance-sensitivephotoresist material.
 18. A substance-sensitive structure for sensingthe presence and/or concentration of a substance in a test medium, saidstructure comprising:structure means, responsive to changes in asubstance-sensitive photoresist layer, for indicating the concentrationof said substance in said test medium; and a substance-sensitive layerdisposed in said test medium and responsive to the concentration of saidsubstance, said structure means comprising a pair of lines coupledthrough said substance-sensitive layer, said substance-sensitive layerincluding a substance-sensitive material and further comprising the solemeans for bonding said substance-sensitive material to said structuremeans.
 19. The structure of claim 19, wherein said structure meanscomprises a field-effect transistor and said substance-sensitive layeris disposed on said field-effect transistor.
 20. The structure of claim23, wherein said field-effect transistor comprises:a semiconductorsubstrate; a pair of spaced-apart diffusion regions on saidsemiconductor substrate; a blocking layer covering said substrate andspaced-apart diffusion regions; and a reference electrode coupled to oneof said diffusion regions, said substance-sensitive layer separatingsaid blocking layer from said test medium and said reference electrodedisposed in said test medium.